Particle size analyses, that is, measurements of the relative proportions by weight of particles of a given sample in different size (diameter) ranges, are widely used in process control and optimization. The size range of a given fraction may be characterized, for example, as being between 0.01 and 0.05 inch, which means that the particles in that range are retained on a screen having openings smaller than 0.01 inch but pass through a sieve having openings larger than 0.05 inch. Such analyses are frequently performed with sieves (screens) of progressively finer mesh sizes, such as the well-known U.S. Standard testing sieves. The sample to be analyzed is placed on the coarsest sieve at the top of a stack of sieves and the entire stack is shaken, particles of different size ranges being retained on different sieves. The sieves are then removed one by one from the stack and the fractions on them are emptied onto a scale and weighed to determine the proportion of the fraction relative to the total sample weight. If the analysis is carried out manually, as is often done, the procedure is slow and labor intensive. A mechanical shaking device such as the "Ro-tap" shaker made by W.S. Tyler, Inc., of Mentor, Ohio, can be used to apply standardized shaking schedules to the stack of sieves, but nevertheless each sieve must be taken manually from the stack, the retained fraction emptied from it and weighed, and the emptied sieves restacked in proper sequence for the next analysis.
In many laboratories and manufacturing processes it is necessary to make particle size range analyses frequently and routinely. It may, for example, be desirable to monitor the proportion of "fines" (particles below some predetermined minimum size), or the proportion of coarse "overs"; or the relative distribution among size ranges may be important for process control. Because manual particle size range analyses are time consuming, in those applications where they must be performed frequently and routinely there exists a need for an automatic particle size analyzer which will separate the various size range fractions, and empty and weigh them individually with no or minimal manual control and manipulation.
Any automatic particle size analysis using a graduated set of sieves necessarily requires separately weighing the particles retained on each sieve. Automatic apparatus for carrying out a separation, and weighing and calculating the fractions is known and is commercially available. The "Gradex" particle size analyzer, which is described in Marrs U.S. Pat. No. 4,487,323 and which is made and sold by the assignee of this application, is one such analyzer. In that apparatus the sample to be analyzed is fed into a horizontal polygonal drum having sieves of progressively coarser mesh sizes on its side faces. The drum is first indexed or positioned rotationally so that the finest sieve is at the bottom, and the sample to be analyzed is deposited on that sieve. The entire drum is shaken, so that particles finer than the mesh of the sieve pass through and fall onto a weigh pan and are weighed automatically by an electronic scale. The drum is then indexed rotationally so that the next finest sieve is at the bottom; the particles retained on the first sieve fall onto the second sieve. The drum is again shaken and the particle fraction which can pass through the second sieve is thereby separated. The process of drum indexing, shaking, and weighing is continued automatically until the sample has been screened on each sieve. The fraction weights may be totaled by a computer and their relative percentages determined and displayed in a readout.
The Gradex machine is expensive and comparatively slow by reason of the polygonal drum which must be indexed to and shaken at each rotational position. Moreover the analyses it provides do not always correlate directly with analyses made with a more conventional stack of standard screens. It has therefore been desirable to provide a machine which is comparatively simpler and faster, and which will provide accurate analyses using a standard stack of sieves rather than a polygonal drum, and which can be operated entirely automatically with virtually no operator attention.